1. Field of the Invention
The invention relates in general to a structure of a multi-stage read only memory (ROM) and a method of fabricating the same, and more particularly to a structure of a multi-stage ROM including memory cells with different threshold voltages and different effective channel widths and a method of fabricating the same.
2. Description of the Related Art
Read only memory (ROM) has been widely used in minicomputers, microprocessors and other digital hardware similar in kind. A ROM can be used to store information of a system, for example, a basic input output system (BIOS). Because of the complexity of processes for manufacturing ROM, much time is spent in each of the manufacturing steps and the pretreatment. Therefore, customers usually deliver programming data to the factory for fabricating the ROMs, where partially completed ROMs are programmed to produce a finished product.
Because most devices of a ROM have the same structure, except for data stored in the programming step, in most integrated circuit (IC) factories, mask ROMs are partially produced without performing programming, and are stocked. After receiving an order for special programming from a customer, a mask is produced to perform a programming step and finish the production, so that the lead time can be reduced to bring better service to customers. Therefore, post-programmed mask ROM is commonly used in the art.
Generally, a channel transistor is used as a memory cell for ROM. In the programming process, a number of channel transistors have impurities selectively implanted into specific channel regions to modify the threshold voltages of the transistors. In this way, the conductivity of the memory cells can be controlled. FIG. 1 is a top view of the integrated circuit layout of memory cells labeled 10, 11, 12 in a ROM. A polysilicon word line (WL) bestrides a polysilicon bit line (BL). The channel 13 of the memory cell 10 is formed under the word line and between the bit lines. The data in binary form is stored in the memory cell by the implantation of impurities in the channel 13.
Presently, for multi-stage read only memory, double code implanting is used to produce different threshold voltages of memory cells to construct a multi-stage logic stage. As shown in FIG. 2, the memory cells are provided with different threshold voltages V.sub.T, V.sub.T1, V.sub.T2 by implanting ions into specific channel regions.
The conventional ROM described above makes use of different threshold voltages to control the logic state of the memory cell simply by implanting ions into the specific channel regions. Because of the capacity limitation of binary programming and the difficulty increasing the density of cells to reduce the overall footprint of the IC, performing a multi-stage process is the only way to improve the memory capacity.